I. Technical Field of the Invention
The present invention relates to an electric power steering system for a vehicle and a four wheeled multi use light traveling vehicle provided with the same and, more particularly, to an electric power steering system for a vehicle provided with an electric power assist device including an electric motor.
II. Description of Related Art
Typical examples of a power assist device of a power steering system for a vehicle include a hydraulic power assist device having a hydraulic pump and an electric power assist device having an electric motor.
At present, the hydraulic power assist device has prevailed, with the advantages of great power performance and freedom of arrangement of hydraulic equipment, such as a hydraulic pump at a desired position by use of a hydraulic hose. However, since periodic inspection of the device for an oil leakage or the like is required, maintenance on the device takes significant effort, and further, the continuous driving of the hydraulic pump increases engine power loss.
In contrast, the electric power assist device is compactly constituted of a small number of component parts in comparison with the hydraulic power assist device, and further, is advantageous due to small engine power loss and saved fuel consumption. Unlike the continuously driven hydraulic pump or the like, the electric power assist device has weak power performance in comparison with the hydraulic power assist device. However, a recent improvement in the performance of the electric motor enhances the power performance, thus growing a demand of the electric power assist device.
The electric power steering system for a vehicle provided with the above-described electric power assist device, more particularly, the electric power steering system for the vehicle having a rack and pinion type steering gear device is classified into mainly three types according to the fixing position of the electric power assist device, as follows.
A first type is a column assist type, in which an electric power assist device is fixed to a steering column; a second type is a rack assist type, in which an electric power assist device is fixed to a rack in a rack and pinion type steering gear device; and a third type is a pinion assist type, in which an electric power assist device is fixed to a pinion gear in a rack and pinion type steering gear device.
FIG. 7 is a view showing an electric power steering system for the vehicle of the pinion assist type disclosed in U.S. Pat. No. 6,988,027. The electric power steering system for a vehicle of the pinion assist type includes a steering wheel 100, a steering column 102 incorporating a steering shaft 101 therein, a steering torque transmitting shaft 112, and a rack and pinion type steering gear device 103. An electric power assist device 110 having an electric motor 111 and the like is fixed to the rack and pinion type steering gear device 103 so as to be connected directly to a pinion gear of the rack and pinion type steering gear device 103.
In the case where the electric power assist device is disposed in an existing steering system for a vehicle having no power assist device, the electric power assist device needs to be disposed in a steering power train from the steering column 102 to the steering gear device 103, as shown in FIG. 7. Here, the electric power assist device requires a large space for the electric motor 111, unlike the hydraulic power assist device. Therefore, it is frequently difficult to secure a space for installing the electric power assist device. Particularly, the space around the steering column and the steering gear device is frequently restricted, thereby making it difficult to dispose the electric power assist device.